1. Field of the Invention
The present invention relates to a feedback control system for controlling the amount of a fluid supplied to an apparatus operable with the fluid such as an internal combustion engine whose operational state varies with the amount of the fluid supplied thereto.
2. Description of Background Information
In an internal combustion engine, for example, a fuel supply control device such as a throttle valve and a carburettor is provided to the air induction system for automatically supplying a suitable amount of the fuel according to the operational state of the engine, so as to obtain a desirable operation of the engine by adjusting the amount of the fuel supplied, in the fluid form, per unit time to the engine. However, from the view point of the emission control, it is difficult to always realize the optimum air/fuel ratio by only utilizing the conventional fuel metering system made up of the throttle valve and the carburettor unit. Therefore, an air intake side secondary air supply system is utilized as the feedback control system of the amount for adjusting the amount of the supply fluid, in which a secondary air passage communicated with the downstream side of the throttle valve for example, is provided and the amount of the secondary air is controlled in response to engine parameters so as to provide more precise adjustment of the air/fuel ratio.
As an example of this type of air intake side secondary air supply system, the applicant has already proposed an air/fuel ratio control system in which an air control valve whose sectional area is variable with the pressure of the fluid applied to a pressure chamber thereof, is provided in the secondary air supply passage, the actual air/fuel ratio is measured by detecting the oxygen content in exhaust gas, a first control pressure for opening the air control valve is applied to the pressure chamber so as to gradually increase the sectional area of the passage when the air fuel ratio is rich, and a second control pressure for closing the air control valve is applied to the pressure chamber so as to gradually decrease reduce the sectional area of the passage when the air/fuel ratio is lean, whereby performing the air/fuel ratio control by integral operations of the pneumatic type. This fluid supply amount control system of pneumatic type is advantageous in that the response characteristic to a fluid supply command is relatively good.
On the other hand, it is known in the art that the so called PI operation, that is, a combination of the proportional operation and the integral operation is preferable in the case of the feedback control system of the fluid supply amount, for improving the response and stability characteristics.
Therefore, in certain air intake side secondary air supply systems of the conventional thechnique which have a function of the PI operation for the air/fuel ratio control, a needle type valve driven by a pulse motor for varying the sectional area of the passage is provided to the secondary air passage, and the rotation of the pulse motor is electronically controlled in accordance with engine parameters.
However, the control circuit for this pulse motor necessarily includes an integration control circuit and a proportional control circuit, and consequently there was a problem of cost up due to rather complicated circuit construction thereof. Futher, the same problem existed also in other various feedback fluid supply control systems.